Color imaging in graphic arts has been well established for many years. This had normally been done in the past by producing halftone plates from images photographed separately through red, green, and blue filters. The negatives serve as the separation positives for the inks. Printing inks used with these plates are typically the cyan, magenta, and yellow subtractive colors. Very often a fourth plate applying black ink is also used. For particularly critical work additional inks may be added. While traditional lithographic printing relies mostly on transparent inks, the inks used to print corrugated container media are normally opaque. With an opaque ink a later printed image that falls on top of one previously printed will usually completely obscure it. By rendering the original negative separations with cyan, magenta, and yellow (CMY) transparent inks, a wide range of reproduced colors is possible. In addition to their individual ink colors, two inks when overprinted give rise to red (magenta and yellow), green (cyan and yellow), and blue (magenta and cyan). With a three ink overprint a black results.
Color printing on packaging has long been considered necessary for attracting ultimate purchasers to the product. If corrugated shipping containers may be used for an example, the brown color of the unbleached kraft papers normally used does not provide an attractive background for color printing. What color is used is typically an opaque black ink, occasionally with one or two other opaque ink colors. These inks are usually used to print simple illustrations consisting mostly of text, lines and solid fields rather than complex natural images that are intimately intermingled as they would be in a halftone image. Some packages are formed of corrugated containerboard that has a thin white outer surface, so-called mottled white, to support more attractive color graphics. Again, printing of these packages calls for simple art consisting of text, lines and solid fields using primarily opaque inks. More sophisticated (and much more expensive) corrugated packaging may have a preprinted coated white paper laminated over most or all of the outer surface to enable higher quality graphics
So-called four color process printing is relatively expensive, both from the standpoint of the presses needed, and the number of inks required. This also has more stringent production requirements; e.g., holding register and controlling density in all four plates. Various investigators have tried to achieve satisfactory color by using simpler methods. Traditional duotone printing is normally done as a halftone using black with one other color. The color provides some improvement in dynamic range but the image is almost entirely carried by the black component. Similarly, two color images have been made using other ink colors, e.g., orange and cyan with very mixed results. Recently, Edwards, in U.S. Pat. Nos. 4,554,241 and 4,998,962,has described a color printing system using superimposed impressions with two different opaque inks. This has been developed commercially into a system called Markcolor by Markolor Imaging Technologies Inc., Livonia, N.Y. It uses an opaque red ink with either an opaque black, green or blue ink. Initial separations are made using exposures through multiple filters. The first separation is made through a red filter followed on the same film by a shorter exposure using a blue filter. The second separation is made through a green filter followed by a short exposure through a blue filter to carry some content in the yellow portion of the spectrum. Separations can now be made using commercially available software such as Adobe Photoshop®, available from Adobe Systems, San Jose, Calif. The resulting two color images approximate the original but with considerable loss of fidelity in some colors.
A recent patent to Power et al., U.S. Pat. No. 5,982,924, describes a model-based algebraic algorithm for printing an image using two inks. The user may select none, one, or both of the colors to be used and enter these choices into the algorithm. The algorithm will extensively sample the available data base of ink colors and choose the best pair of inks for rendering the reproduction, based on minimization of color error between the original and reproduction. It will then suggest additional “best pairs” if requested by the user. Out of gamut colors in the original are mapped onto the smaller gamut defined by the given ink pair. The method takes considerable computing power but leaves some user discretion as to the final results achieved.
To date there has been no truly satisfactory method for approximating a full color image using only two inks The present invention provides a superior solution to this problem using a simple method employing readily available software.